This invention relates to a structure of a steam introducing part of a steam turbine, which part introduces main steam into a nozzle box through turbine casings.
In a main steam introducing part of a large capacity steam turbine, an inner casing is provided in an outer casing, and a nozzle box is disposed in the inner casing. A main steam passage passes through the outer and inner casing walls and communicates with the nozzle box, whereby the main steam is fed to the nozzle box through the main steam passage. In the portion of the inner and outer casings through which the passage passes, an expansion joint is provided to allow thermal expansion or contraction and to introduce main steam from the outer casing into the nozzle box.
The above-mentioned nozzle box has an annular shape as a whole, and is of two kinds, one of which has a chest of the first stage nozzle which is divided circumferentially into four 90.degree. type nozzle boxes and the other two 180.degree. type nozzle boxes.
The above-mentioned 90.degree. type nozzle box, which is an early type, has a steam pipe portion which is a part of the nozzle box and extends to the outer casing through the inner casing for introducing main steam into the nozzle box. The nozzle box is fixed to the inner casing at the position that the steam pipe portion passes through the inner casing. Therefore, the nozzle chest which faces the first stage moving blades of a rotor is hung by the inner casing. This structure has an advantage that the nozzle box can freely extend without any restriction on the thermal expansion. The 90.degree. nozzle box also has disadvantages that it is unstable to the nozzle reaction and microvibrations caused by the steam flows during the operation of the turbine.
According to a mounting method, the 90.degree. nozzle box has two types one of which is fixed to the inner casing by utilizing the steam introducing pipe portion, the other fixed to the inner casing by fastening the chest portion with bolts from the inner casing side so that the chest portion is hung by the inner casing as above-mentioned.
The 90.degree. nozzle box has the above-mentioned disadvantage, so, recently, the 180.degree. nozzle box is used. This nozzle box is divided into two parts by a horizontal plane at the first stage nozzle chest portion, the lower half nozzle box is supported on the horizontal face of the inner casing and the upper half nozzle box is fixed to the lower half nozzle box by bolts. The construction is not restricted to the expansion, what's more, it is stable even when the nozzle reaction and microvibrations take place due to steam flows.
The steam introducing part of the turbine with 180.degree. nozzle box is provided with a steam conduit which is independent of the inner and outer casing and the nozzle box, passes through the inner casing and is engaged with the outer casing and the nozzle box at the end portions. At the portions of the inner and outer casings and the nozzle box, through which the steam conduit passes through, seal rings are provided to prevent steam leakage.
Recently, a preventive maintenance activity for raising the reliability of turbine parts or elements has been activated, so, change of a turbine with 90.degree. nozzle box to a turbine with 180.degree. nozzle box has been put into practice.
The above-mentioned change of turbines from a turbine with 90.degree. nozzle boxes to one with 180.degree. nozzle box is difficult according to types of the 90.degree. nozzle boxes.
Improvement of a used steam turbine with 90.degree. nozzle box by changing it to a steam turbine with 180.degree. nozzle box is disclosed in "Hokkaido Thermal and Nuclear Power Generation News" issued by Thermal and Nuclear Power Generation Technique Association Hokkaido Branch on March of 1983, paragraph 4 of the nozzle box and diaphragm item, pp 8. According to this literature, the improvement can be effected by newly manufacturing the 180.degree. nozzle box and modifying the inner casing. This improvement, however, is concerned with only a turbine with the above-mentioned 90.degree. nozzle box fixed to the inner casing by making use of the steam introducing pipe portion as mentioned above.
As for the used turbine with the 90 nozzle box fixed by bolts at its chest portion, the improvement of the turbine by changing the 90.degree. nozzle box to the 180.degree. nozzle box is not put into practice, for the reasons that the steam introducing pipe and the 180.degree. nozzle box can not be enclosed in the inner casing by only modifying the inner casing, and that the nozzle box, the steam introducing pipe and the inner casing are necessary to produce new products, so that the improved or reconstructed turbine will be high in cost.